Simple!
Maybe it should detect if we are in dark mode and display the count in white.

EDIT: Good improvement on the customization, numbers are still a bit little (the size of the icon don't get bigger, maybe put one number on top of the other number?), but over all it's an excellent extension.

The current tab index is a must over all other tab counters addons as it is very nice to know where we are in the middle of all those tabs

I'am using Firefox 70 on Ubuntu-MATE

Thanks Jonathon